1. Field of the Invention
The present invention relates to a stacked piezoelectric resonator and more particularly, the invention relates to, for example, a stacked piezoelectric resonator including a base having piezoelectric layers and internal electrodes and that is constructed to vibrate in a longitudinal vibration mode, a characteristics-adjusting method therefor, and a ladder-type filter including such a resonator.
2. Description of the Related Art
A conventional stacked piezoelectric resonator of the above-mentioned type related to the present invention is disclosed in, for example, Japanese Unexamined Patent Publication No. 104330. FIG. 10 is a view of a conventional stacked piezoelectric resonator 1 which includes a base 2 having a rectangular-parallelepiped shape. The base 2 is formed by stacking multiple piezoelectric layers 3 and multiple internal electrodes 4. Stacked surfaces of the piezoelectric layers 3 and the internal A electrodes 4 are arranged perpendicular to a length direction of the base 2. On a peripheral surface of one of two sides of the base 2, exposed portions of the internal electrodes 4 are covered alternately by insulators 5a. On a peripheral surface in the other one of the two sides of the base 2, exposed portions of the internal electrodes 4 which are not covered by the insulators 5a are alternately covered by insulators 5b. In addition, external electrodes 6a and 6b are provided on one of the two sides and the other one of the two sides of the base 2. Since the insulators 5a and 5b are provided on the two sides, the internal electrodes 4 arranged to be adjacent to each other are connected alternately to the external electrodes 6a and 6b. In such a conventional stacked piezoelectric resonator 1, the piezoelectric layers 3 arranged to be adjacent to each other are polarized in opposite directions relative to each other in the length direction of the base 2, as indicated by arrows in FIG. 10.
To manufacture the stacked piezoelectric resonator 1, a mother substrate 7 shown in FIG. 11 is prepared. The mother substrate 7 is formed by stacking the multiple piezoelectric layers 3 and the multiple internal electrodes 4. As indicated by dotted lines in FIG. 11, the mother substrate 7 is cut perpendicular to the stacked surfaces. The mother substrate 7 is thus cut, thereby forming the base 2. Also, the insulators 5a and 5b are disposed on opposite peripheral surfaces of the base 2, and the stacked piezoelectric resonator 1 is completed by burning processing performed after materials for the external electrodes are coated on the base 2.
In the conventional stacked piezoelectric resonator 1, in response to input of signals to the external electrodes 6a and 6b that define input/output terminals, electrical fields opposing each other are generated between the internal electrodes 4. Also, since the adjacent piezoelectric layers 3 are polarized so as to oppose each other, the entire base 2 vibrates in a longitudinal vibration mode according to the electrical fields applied thereto. The stacked piezoelectric resonator 1 thus manufactured can be used as a piezoelectric resonator.
A ladder-type filter is constructed by connecting multiple stacked piezoelectric resonators in series and/or in parallel. In such a ladder-type filter, the amount of attenuation can be adjusted according to adjustment of the capacitance ratio of stacked piezoelectric resonators to be connected in series and stacked piezoelectric resonators to be connected in parallel. For this reason, in the ladder-type filter, the stacked piezoelectric resonators to be connected in parallel have a capacitance that is larger than capacitance of the stacked piezoelectric resonators to be connected in series in order to obtain desired characteristics. The capacitance of the stacked piezoelectric resonators must be adjusted to adjust characteristics of the ladder-type filter. The capacitance of the stacked piezoelectric resonators can be adjusted by, for example, a method for varying the number of the internal electrodes provided in the base, changing distances between the internal electrodes, or permittivity of the piezoelectric layers themselves.
However, there are cases where implementation of the adjustment as described above requires design modification for the mother substrate. Also, even when the capacitance of the stacked piezoelectric resonators is adjusted, there are cases where outer dimensions cannot be changed because of restrictions in construction.